1. Field of the Invention
The present invention relates to a curing apparatus of ultraviolet radiation type and more particularly to an apparatus for curing an ultraviolet-radiation-curable resin coated over the surface of a line-shaped medium by the continuous irradiation of ultraviolet radiation. More specifically, the present invention relates to an apparatus for curing an ultraviolet-radiation-curable resin coated over the surface of an optical fiber drawn into a fine line.
2. Description of the Prior Art
One of the apparatus for curing by irradiating ultraviolet radiation now available on the market is an apparatus manufactured by Fusion Systems Corporation, MD, U. S. A. The apparatus has a magnetron which excites a discharge lamp having no electrode to emit ultraviolet radiation. This apparatus is provided with a cavity having an elliptical reflecting mirror. The discharge lamp is disposed along a first focal axis of the elliptical reflecting mirror, so that the lamp is excited by microwaves transmitted from the magnetron. The ultraviolet radiation emitted from the discharge lamp is reflected by the reflecting mirror toward its second focal axis. A line-shaped medium such as an optical fiber is disposed along the second focal axis, so that the reflected ultraviolet radiation is focused on the medium. As a result, an ultraviolet radiation-curable resin coated over the surface of the line-shaped medium is cured.
Japanese patent application Laying-Open No. 55-152567 discloses a curing apparatus of ultraviolet radiation type as shown in FIGS. 1 and 2. In this apparatus, a source for irradiating ultraviolet radiation is disposed along a first focal axis of an elliptical reflecting mirror and a line-shaped medium coated with ultraviolet radiation (UV) resin which is cured by ultraviolet radiation is disposed to pass a second focal axis of the reflecting mirror. Therefore, ultraviolet radiation is focused on the surface of the line-shaped medium so that the coated ultraviolet radiation resin is cured.
More particularly, referring to FIG. 1, reflecting mirrors 1A and 1B are accommodated in a housing 2. Each of the reflecting mirrors 1A and 1B has an elliptical mirror surface having a first focal axis 3 and a second focal axis 4. A source for irradiating ultraviolet radiation such as a high-pressure mercury lamp 3A is disposed along the first focal axis in such a way that the axis of the lamp 3A coincides with the first focal axis. A transparent quartz tube 4A is disposed along the second focal axis in such a way that the axis of the quartz tube 4A coincides with the second focal axis.
Therefore, ultraviolet rays emitted from the lamp 3A to all directions are focused on the surface of a line-shaped medium 5 such as an optical fiber passing through the quartz tube 4A in the axial direction thereof.
A gas such as nitrogen gas not containing oxygen is introduced into the quartz tube 4A through a pipe 6 so that adverse effects of oxygen upon the passing medium 5 is prevented when the ultraviolet radiation resin is cured in the quartz tube 4A. The line-shaped medium 5 coated with an ultraviolet radiation resin is transported through the quartz tube 4A from a supply tube 7 shown in FIG. 2 at a predetermined speed.
The curing apparatus further comprises a blower 8 and an exhaust fan 9 so that the reflecting mirrors 1A and 1B and the ultraviolet radiation source 3A are prevented from being raised to high temperatures. That is, air is forcibly circulated by the blower 8 and the exhaust fan 9 for cooling the inside of the reflecting mirrors 1A and 1B. The ultraviolet radiation lamp 3A is connected to a power supply (not shown) through a terminal 10.
In the case of a curing apparatus of the type utilizing a magnetron, an electrode-less lamp is used as the source of ultraviolet radiation, so that the ultraviolet radiation source has a long lifetime and it is possible to reduce the diameter of the ultraviolet radiation source. However, the position of the ultraviolet radiation source is inevitably determined depending upon conditions to form a cavity as a part of magnetron, while the second focal axis of the opposite reflecting mirror is determined depending upon conditions as a reflecting mirror. As a result, the distance between the first and second focal points becomes long, so that ultraviolet rays are not sharply focused at a point in the vicinity of the second focal point.
Furthermore, in order to provide the ultraviolet radiation source over a predetermined distance in the direction in which the line-shaped medium 5 is transported, a plurality of magnetrons must be disposed in the transportation direction. Moreover, these magnetrons as well as the source of ultraviolet radiation must be cooled. As a result, the curing apparatus becomes large in size.
As described above, according to Japanese patent application Laying-Open No. 55-152567, the ultraviolet radiation source 3 is disposed at the first focal point 3 in the manner described above and the line-shaped medium 5 is positioned to pass through the quartz tube 4A at the second focal point 4 in the manner described above. The results of the extensive studies and experiments conducted by the inventors show that the energy of ultraviolet radiation focused on the surface of the line-shaped medium 5 is dependent upon the angles of reflection and incidence. Furthermore, some ellipsoidal reflecting mirrors cannot satisfactorily converge the energy of ultraviolet radiation. As a result, there arise the problems that a resin coating cannot be cured uniformly and that the curing rate is relatively slow.
In order to increase the curing rate, a plurality of ultraviolet radiation sources must be provided and it is necessary to device counter-measures for improving the focusing efficiency of the ultraviolet rays. For instance, the angles of reflection and incidence must be suitably adjusted. The reflecting mirrors must be coated with a special coating material so as to increase their reflectivity. However, in view of the limitations imposed by the volume and weight of a curing apparatus, the cost thereof and the lifetime and stability of the ultraviolet radiation source, such counter-measures as described above cannot attain satisfactory result.